Porous membranes are often used in conventional lateral flow and flow-through cartridges, in which flow of fluid occurs by wicking through the membrane (either laterally or transversely) onto an absorbent pad. Immunoassays take advantage of porous wick systems to measure and analyze analyte samples. The dependence on wicking to generate flow greatly limits control over assay conditions. Specifically, lateral flow assays are often limited to a single step in which sample (and buffer) is added to the sample pad, and the sample flows by capillary action (i.e., wicking) along the pad. Capillarity provides the force needed to provide a nearly continuous flow of fluid from one point to another, causing reagents stored in dry form to be transported along the device and to pass over regions that contain immobilized capture molecules. These devices are restricted to simple one-shot detection chemistries like colored nanoparticles that do not provide the sensitivity possible with multistep-detection chemistries, such as enzymatic amplification.
Microfluidic systems that include open fluid channels for the flow of buffers, samples, and reagents can inherently be made much more sophisticated, and it is possible to use them to carry out a very large number of fluid-processing steps. Such microfluidic systems usually incorporate a complex disposable, which leads to unavoidably high per-test manufacturing costs and the need for expensive external pumps and valves to move fluids. While microfluidic devices can inherently be very flexible in the functions that they perform, they are also inherently complicated and expensive. Additionally, the devices that have been made that support complex function are usually quite complex themselves. For example, some polymeric laminate cartridges currently developed contain as many as 23 different layers, each of which must be separately manufactured and bonded to the others.